Longest Consecutive Sequence

Question:

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Your algorithm should run in O(n) complexity.

Solution:

class Solution {
public:
    int longestConsecutive(vector<int>& num) {
        map<int,bool> used;
    for(vector<int>::const_iterator iter=num.begin();iter!=num.end();iter++)
    {
        used[*iter]=false;
    }
    int longest=0;
    for(vector<int>::const_iterator  ptr=num.begin();ptr!=num.end();ptr++)
    {
        if(used[*ptr])
            continue;
        int max_length=1;
        used[*ptr]=true;
        for(int j=*ptr+1;used.find(j)!=used.end();j++)
        {
            used[j]=true;
            max_length++;
        }
        for(int j=*ptr-1;used.find(j)!=used.end();j--)
        {
            used[j]=true;
            max_length++;
        }
        longest=max(longest,max_length);
    }

    return longest;
    }
};